I boiled it for about an hour and let it soak in a bucket to get all the crap out of it for about 2 weeks and still floats like a champ. Maybe i'll try using a larger drill and to create a pocket for the head of the screw. Didn't really think of that. Slate is so soft it shouldn't be hard to do. And the piece is a large so zip ties would look bad judging by the number i would need to sink it.
Thanks guys